Optical fibres comprise a lightguide which, as its name suggests, has the purpose of conducting light waves.
When this lightguide is made of silica, it is fundamental to protect this silica from being attacked by the chemicals normally or accidentally present in the external environment. In particular, moisture (or more precisely, OH.sup.- ions) embrittles the silica or quartz of the optical fibres by penetrating the micropores inherent in its crystalline structure, thus degrading the optical properties of the lightguide. The chemical attack on the silica, accelerated by the mechanical (bending, tensile, etc.) stresses to which the optical fibre is subjected, over time produces fracture initiators on the lightguide.
Various protective coatings have been proposed so as to remedy these drawbacks. Thus, it is known to provide silica optical fibres with a coating made of an epoxy acrylate resin. This coating is applied immediately after drawing the optical fibre, or even simultaneously therewith. It prevents the drawn quartz from oxidizing, blocks the micropores present in the fibre and increases the mechanical strength of the fibre.
However, the epoxy acrylate has the drawback of being permeable to OH.sup.- ions, and is therefore water-soluble, and to moisture, which results in the destruction or blistering of the epoxy acrylate protecting the fibre, the protection against the external environment then losing its effectiveness over time.
Consequently, the optical fibre coated with an epoxy acrylate is normally placed in microtubes internally coated with a petroleum jelly so as to protect it from the undesirable effects of moisture.
However, such a structure has an extremely poor mechanical strength because of the presence of the protective microtube, which is moreover subject to folding and has an elasticity which lends itself poorly to effective protection of the fibre.
Furthermore, the protective system consisting of the microtube internally coated with petroleum jelly does not allow a very high fibre density (namely, a large number of fibres per cable) to be obtained, because of the diameter of the microtube which it is necessary to adopt in order to allow the jelly to be deposited. By way of example, it is enough to point out that, for an optical fibre only 250 .mu.m in diameter, the tubes have a diameter of 1.5 to 2.5 mm.
It has also been proposed to coat the optical fibre with a film of UV-crosslinkable silicone, as described in Patent FR 2,628,847. However, silicones have a strong affinity for water. It is therefore necessary to provide an external sheath made of a rigid plastic, for example polyetherimide. This external sheath has the drawback of increasing the diameter of the fibre up to 1 mm, a diameter which, once again, does not allow a very high fibre density to be obtained in cables.
In addition, since silicones are pollutants, they have to be recycled after the fibres have been used.
A drawback common to all these solutions of the prior art is the incompatibility between the quartz of the optical fibre and the coating used for protecting the latter, resulting in insufficient adhesion of the protective coating, which is applied in a loose manner to the optical fibre.